Heat treating magnesium alloys



Patented Oct. 30, 1928.

1,689, UNITED STATESPATENT oFFIc zAY warms AND 301mm s ARCHER, or CLEVELAND, omo, AssIGNoRs TO'AMERI- CAN MAGNESIUM CORPORATION, or NIAGARA FALLS, NEW YORK, A conroR 'rIo v or NEW roar.

HEAT TREATING MAGNESIUM ALLOYS.

Application filed. October 14, 1921. Serial .N'o. 507,766.

No Drawing.

Our invention relates to the treatment pf magnesium-base alloys, that 1s, those in which the principal metal is magnesium.

metal during the heat treatment. With these and other objects in view, our invention consists in certain novel features as will be hereinafter described and particularly pointed out in the claims.

As illustrative of our discovery, we take a magnesium-base alloy casting contalmng from 3 to 15% aluminum and which may contain in addition thereto, small quantities of other metals such as zinc up to 5%, cadmium up to 10%, copper up to 2%, manganese up to 2%. Such alloy may be in the form of a casting and the casting may be any of the well known types,.e. g. green sand, dry sand, semi-permanent mold, permanent mold or die casting.

In carrying out our invention, the metal is subjected to heat treatment at a temperature slightly below the melting point of the most fusible constituent of the alloy for an extended period of time, sometimes as long as hours. With the alloys hereinabove specified the most suitable temperature is between 400-440 degrees C. or about-1050 degrees C. below the fusing point of the .magnesium-aluminum eutectic richest in magnesium.

Heat treatment for shorter periods of time improves the properties of the alloy but not to the extent of longer heat treatment. The effect of this heat treatment may be indicated by the following example. Magnesium castings containing approximately 5 to 7% aluminum, have a tensile strength after casting in green sand of about 22,000 lbs. per sq. in. and an elongation of 1 to 3%. After heating for 48 hours at a temperature of 420-430 degrees C. we have secured a tensile strength above 30,000 lbs. per sq. in. and an elongation of more than 8% in two inches. Magnesium-base alloys containing from 3 to 15% aluminum, with or Without small additions of other elements, are particularly C. or above in an air atmosphere may produce spontaneous burning of the metal. This is more noticeable in some types of alloys than in others and frequently the heat treatment is satisfactory when carried out in an air atmosphere. en it is desired, however, to nsure against any trouble from this source, we prefer to put the metal in an iron or other suitable container and maintain an inert atmosphere or vacuum in this container while the metal is at heat. The theory upon which magnesium-base alloys are improved by heat treatment maybe explained in part as follows:

' In the casting of alloys composed principally of magnesium and containing other alloylng metal, soluble, at least within a certain range of composition, in the magnesium metal in the solid state, the saturation quantity of alloying element is not in solution in the magnesium after casting. Taking the magnesium-aluminum alloys, as an example, the equilibrium diagram would indicate that about 11% of aluminum by weight should be soluble in magnesium at the freezing temperature of the eutectic alloy. In a magneslum casting containing as little as 4 or 5% aluminum a considerable quantity of an aluminum-richconstituent, separates out in a complete or partial net work enclosing, or adjacent to, grains of magnesium-rich constituent. We have discovered that .the plasticity of the metal particularly suffers because of this structure and the strength is also relatively low.

An equalization of composition can be ef-' fccted by heating the castings for a relatively long period of time, sometimes requiring up to seventy hours and we have discovered that the physical properties are very much improved by this heat treatment process.

While We refer herein to the application of the process to castings We do not intend to so limit our discovery, as the same heat treatment could be applied to articles after they have been mechanically worked or alloy while hot.

2. The process of working a magnesium base alloy containing a quantity of aluminum capable of dissolvlng in solid magnesium 1n an amount greater than'is in solution in the alloy as cast, comprising subjecting the alloy to a heat treatment at a high temperature but below the melting temperature of the most fusible constituent of the alloy to cause increased solution of the aluminum in the solid magnesium, thereby increasing the plasticity of the alloy and then mechanically deforming the alloy while hot.

3. The process of working magnesium base alloys containing from 3 to 15% aluminum and capable of dissolving in solid magnesium up to about 11% aluminum and in an amount greater than is in solution in the alloy as cast, comprising subjecting the alloy to a heat treatment at a high temperature, but below the melting temperature of the most fusible constituent of the alloy to cause in-.

creased solution of the aluminum in the solidmagnesium, thereby increasing the plasticity of the alloy and then mechanically deforming the alloy while hot.

4. The process of working magnesium base alloys containing a quantity of aluminum capable of dissolving in solid magnesium in an amount greater than is in solution in the al loy as cast comprising subjecting the alloy to a heat treatment at 400440 C. for a period of hours to cause increasedsolution of the aluminum in the solid magnesium, thereby increasing the plasticity ofithe alloy and ilhen mechanically deforming the alloy while 0t. 5. The process of working magnesium base alloys containing 3 to 15% aluminum and capable'of dissolving in solid magnesium up to about 11% aluminum and in an amount greater than is in solution in 'the alloy as cast, comprising subjecting the alloy to a heat treatment at 400-4AO 'C. for a period of hours to cause increased solution of the aluminumin the solid magnesium, thereby increasing the plasticity of the alloy, and then mechanically deforming the alloy while hot.

' 63 The process of working magnesium base alloys, comprising subjecting the solid metal to heat treatmentat a high temperature for a period of hours but below the melting temperature of the most fusible, constituent of the alloyto increase the plasticity of the alloy and thereafter mechanically deforming the alloy. v

In testimony whereof they aflix their signatures.

' ZAY JEFFRIES.

ROBERT S. ARCHER. 

